There are various different notions measuring extremality of hypergraphs. In this survey we compare the recently introduced notion of the codegree squared extremal function with the Turán function, the minimum codegree threshold and the uniform Turán density.
The codegree squared sum co2(G) of a 3-uniform hypergraph G is defined to be the sum of codegrees squared d(x,y)2 over all pairs of vertices x,y. In other words, this is the square of the ℓ2-norm of the codegree vector. We are interested in how large co2(G) can be if we require G to be H-free for some 3-uniform hypergraph H. This maximum value of co2(G) over all H-free n-vertex 3-uniform hypergraphs G is called the codegree squared extremal function, which we denote by exco2(n,H). We systemically study the extremal codegree squared sum of various 3-uniform hypergraphs using various proof techniques. Some of our proofs rely on the flag algebra method while others use more classical tools such as the stability method. In particular, we (asymptotically) determine the codegree squared extremal numbers of matchings, stars, paths, cycles, and F5, the 5-vertex hypergraph with edge set {123,124,345}.
Additionally, our paper has a survey format, as we state several conjectures and give an overview of Turán densities, minimum codegree thresholds and codegree squared extremal numbers of popular hypergraphs.